Dangerous waves and your boat

Mar 31, 2011

Steve Tredup

At what size do waves get big enough to cause real trouble for your boat?

by Steve Tredup

It’s dark. It’s storming. The waves, when you can see them, look large. And you are sailing a long way from land. One of the greatest concerns a sailor may have is that he or she will be caught in a situation where his boat may be knocked down or rolled by a wave.

Despite being frightened by stories about exceptionally dangerous storms, such as those that wreaked havoc on the 1979 Fastnet or 1998 Sydney to Hobart races, not all storms create dangerous breaking waves. But some storms do and a prudent sailor has to ask: Do I know what wave heights are dangerous to my boat? Do I know when breaking waves can occur? Do I know when these conditions might occur where I am sailing?

This is not about heavy weather sailing techniques, but knowing which waves caused by heavy weather may be dangerous.

Roll, roll, roll, your boat
When is your boat in danger of being knocked down or rolled by a wave? A study conducted by the University of Southampton’s Department of Ship Science and a similar study by the Society of Naval Architects and Marine Engineers (SNAME) along with the United States Yacht Racing Union (now US Sailing) have shown that three conditions usually have to exist:

1. Wave height must exceed a certain percentage of the boat’s length. At this point the wave becomes dangerous and contains enough energy to overcome a boat’s righting moment.

2. The boat is broadside or oblique to the wave. The more a boat moves from being perpendicular to a wave, the greater the chance of a knock down. If the boat is bow or stern onto the wave, the chances of being rolled are greatly reduced. Yes, a boat can be pitch-poled (tossed end-over-end), but the size of the wave needed to do this greatly exceeds the size of the smaller wave needed to knock a boat down when broadside to a wave.

3. The boat is struck by a breaking wave. Unless the wave is breaking, the boat should ride up and over the wave regardless of the wave height in comparison to the boat length and the boat’s orientation to the wave. That is not to say that it will be comfortable or possibly cause the boat to go out of control. But without the wave breaking on the boat, the boat itself should not be knocked down.

An ocean wave is energy. Along with the speed of the wave, the larger the wave is, the more energy it will contain. Therefore, the wave has to be of a certain height in order to have enough energy to roll a boat of a certain length and displacement.

Most sailors would be surprised at the minimal wave height needed to roll a boat. Andrew Claughton (who co-authored the University of Southampton, Department of Ship Science’s report) writes in Adlard Coles’ Heavy Weather Sailing by Peter Bruce, “During the model tests that were carried out to investigate the problem, when the breaking wave was 30 percent of the hull length high, from trough to crest, it could capsize some yachts, while waves to a height of 60 percent of the hull length comfortably overwhelm all of the boats we tested.” So, the starting point for a wave to be dangerous to rolling a boat is one that is only 30 percent high as the boat is long.

In order to be rolled, a boat’s righting moment has to be overcome. Obviously, different boats will be more susceptible to rolling than others based on length, beam, displacement, roll inertia, and center of gravity. A monohull’s resistance to heeling is created by its centers of gravity and buoyancy. This resistance to heeling is called a boat’s righting moment, which is created by the horizontal distance from the boat’s center of gravity and the center of buoyancy as the boat tilts. The farther apart horizontally these points are, the more righting moment the boat has.

As a boat tilts farther over, it will eventually reach its maximum righting moment when the maximum horizontal distance between the centers of gravity and buoyancy is reached. If the boat tilts over more, the boat still has a positive righting moment but it will be decreasing. It will continue to decrease until the center of gravity and the center of buoyancy line up again, but the wrong way, which is known as the angle of vanishing stability (often beyond 90 degrees). The boat could then fall either way, but once the point of vanishing stability is passed, the boat will continue over until a new equilibrium is found, which could be a complete roll or an inverted position.

The wave will have to be breaking along its crest as it encounters the boat to roll the boat. At this point, the boat will be at the top of the wave face and will be tilted over by the steepness of the wave. When the breaking front of the wave hits the boat, it acts like a power force to throw the boat over, down the wave into the trough. How far the boat is knocked down depends on the energy of the wave and the righting moment of the boat.

Your boat may be more or less susceptible to rolling than other boats based on its design, but the aspects to remember are that although your boat may be severly tilted over by the wave front, the wave will have to be over 30 percent of the boat’s length, be breaking on the boat, and the boat will have to be orientated obliquely (beam on is the worse) to the wave to knock the boat down or completely roll the boat over. Unless the wave is so large that it pitch-poles the boat, a boat that is bow or stern onto the wave should not be rolled.

Wave background
Local wind causes waves and as these waves get separated out of a localized wind, it is called swell. Swell from outside an area and local wind waves often interact. When a crest meets another crest, the result is the combined height of both. For example, the crest of a five-foot swell and the crest of four-foot wave would stack, resulting in a nine-foot wave (for a wave to double like this, it requires that both waves have very long and regular swells). However, if the crest meets a trough, then they cancel. In our example, this would result in a one-foot wave. All ranges in between will also be encountered.

Because of these interactions, waves will appear different from one another and may seem confused. Waves will appear across a large spectrum of sizes, although to a local observer, there will appear to be a general height. This is why wave forecasts, such as from the National Oceanic and Atmospheric Association, give a range, such as six to eight feet, covering a broad area (called a wave field).

This range, though, is often misunderstood. First, the wave height given is the significant wave height, which is the average of the highest one-third of the waves that may be encountered. It is not the average of all waves, nor will all waves be in this range. Some waves will be smaller and some waves will be higher. In fact, you can expect to see a wave almost twice as high (!) as the significant wave height. This type of wave is rare, however, only showing up once every 3,000 waves.

Wave lesson
Predicting dangerous breaking waves is complex. The best information that most sailors will have to rely on is the weather report. Therefore, when you are sailing, the wave forecast portion of the weather report is just as important as the rest of the report. The wave height (from bottom of the trough to the top of the crest) and the wave length (the distance from one crest to the next crest or one trough to the next trough) are the two pieces of critical information. Once you know the wave height and length, you can determine whether the waves will have the potential to be breaking or not in deep water.

A wave will collapse, or break, in deep water depending on the wave’s steepness. The base of the wave (trough to trough, i.e., the wave’s length) can only support a wave of a certain height. If it exceeds that height, the wave becomes too steep and it collapses and breaks forward.

There are some varying estimates of the scientific, precise moment when the wave will collapse, but generally, when the wave height exceeds the wave length at a 1:7 ratio, it may begin to collapse. This is called the wave’s breaking point, but I call it the “Rule of 7 or Less,” if the wave length is 7 times or less than the wave height, then the wave may break. This is true for any measurement type, feet or meters. For example, if the wave height is 15 feet, multiplying that by 7 will give a minimum wave length of 105 feet. If the wave length is 105 feet or less, then the wave has the potential to be breaking.

Is there danger?
To know if you have entered a wave-height danger zone, you must first know what wave height is dangerous to your boat. Remember, a wave will have enough potential energy to knock your boat down starting at 30 percent of its length and certainly will be large enough if it is 60 percent of the boat’s length.

Using the weather forecast or manual prediction, you need to know the wave heights and lengths you will be expecting to encounter over the next day or so. Will the wave heights exceed your danger zone? Could another condition, such as more wind or a contrary current, suddenly make normally safe waves dangerous?

If yes, then you need to know if the waves will be breaking. If the wave length is 7 times or less than the wave’s height, then you should take precautions.

For example, using the minimum 30 percent wave height to boat length, if your boat is 40 feet long, then the wave-height danger zone starts at waves 12 feet high (40 x 30). A wave 12 feet high could knock down a boat 40 feet long if the boat is caught somewhere near beam onto the wave as it breaks. Following the Rule of 7 or Less, you know that a 12-foot wave could break only if the wave length is less than 84 feet (12 x 7).

In contrast, these conditions should not present a danger to a 40-foot boat even though some may find the below conditions worrisome.

If the wave is higher than 12 feet, but is not breaking (a non-breaking wave cannot throw the boat down into the trough);
If the 12-foot-high wave’s length exceeds 84 feet (the wave will not be breaking); or
If the wave is breaking, but the wave height is less than 12 feet (too small to overcome the boat’s righting moment).

To determine dangerous wave heights and lengths for your boat, you can use these formulas:

Danger Wave Height
W Height = B Length x 30%

Danger Wave Length
W Length < = W Height x 7

However, to make it easier, the accompanying chart (pg. 47) is designed for boats 30 to 60 feet based on a dangerous wave height of 30 percent of the boat’s length.

Knowing the wave conditions that may be a danger to rolling your boat will allow you to take action to avoid or prevent it.

Steve Tredup is a sailor who works in the insurance industry. Tredup lives near Chicago and sails on Lake Michigan.